I'm not sure it is possible. When you preview, the LaTeX code is sent to the server which compiles it and creates the images with the formulas that appear in your posts. Then, the images are sent to you so that you can see them. A real time preview would require that this happens each time you type a character : it might cost much CPU time (because of compilation) and bandwidth. (because of the images)
Anyway, with time, you'll improve your LaTeX skills and won't need to preview as often as now.